1. Field of the Invention
The present invention relates to testing of demodulators in data reception systems and, more particularly, to built-in testing of data demodulators in satellites independent of uplink characteristics.
2. Description of the Prior Art
Satellite data transmission systems are in wide spread usage. These systems suffer from an inability to determine the source of malfunctions within the data transmission uplink. While an erroneous data stream can be detected with a retransmission from a satellite which indicates a malfunction in the uplink, it is very difficult to determine the source of the malfunction from the ground in view of the inaccessibility of the satellite electronics for testing. Currently, determination of the source of a malfunction requires backtracking from the detection of erroneous data in a retransmission from the satellite with there being no current methodology by which the satellite onboard electronics in the data demodulator can be tested conveniently from the ground to determine if malfunctions exist in the demodulator.
FIG. 1 illustrates a block diagram of a conventional demodulator 10 of the type used in data transmission satellites. The demodulator 10 receives an input signal IF.sub.in which has been shifted down in frequency by the satellite to an intermediate frequency. The input signal IF.sub.in containing data is applied to an analog to digital converter 12 which digitizes the input signal IF.sub.in into an output containing a large number of samples which are inputted to a tuner 14. The tuner 14 processes the digitized data outputted by the analog to digital converter 12 into quadrature signal processing paths 16 and 18 which each contain a frequency converter 20 which downwardly shifts the input signal IF.sub.in to a lower frequency. The frequency converters 20 of the I signal processing path 16 and the Q signal processing path 18 respectively receive input carriers COS(.omega.T) and SIN(.omega.T) from the quadrature digital sinewave generator which cause the frequency converters to produce the quadrature I and Q signals which were downshifted in frequency to a lower carrier frequency. The input to the quadrature digital sinewave generator 22 is a frequency command F.sub.in which commands the quadrature digital sinewave generator 22 to output the quadrature carriers COS(.omega.T) and SIN(.omega.T) of the appropriate frequency to cause the frequency converters to shift the input signal IF.sub.in to the lower carrier frequency for further signal processing. The envelopes of the lower frequency quadrature carriers produced by connection of COS(.omega.T) and SIN(.omega.T) to the frequency converters 20 are modulated with the quadrature components of data present in the intermediate frequency input signal IF.sub.in. The outputs from the frequency converters 20 are applied to suitable low pass filters 24 which attenuate frequency components outside the desired lower carrier frequency band to which the I and Q data components are shifted. The output I and Q signals are applied to downstream demodulator processing 26 of a conventional nature including channelization, discrete Fourier transformation (DFT) and other known signal processing techniques.
As has been stated above, a demodulator, including a tuner 14 in accordance with the prior art of FIG. 1, is not readily diagnosed for malfunctions occurring downstream of the tuner. This seriously affects the ability to locate where processing errors occur when the output transmissions of a data satellite contains erroneous data.
FIG. 2 illustrates a block diagram of a preferred embodiment of a demodulator containing a digital tuner 30 of the type used with the practice of the present invention. It should be understood that separate I and Q channels are present in FIG. 2 but have been omitted to simplify the illustration. The digital tuner 30 receives an intermediate frequency data input IF.sub.in like that of FIG. 1 which is applied to an analog to digital converter 32 which performs the same function as the analog to digital converter of FIG. 1 to sample the data into a large number of data samples. The output of the analog to digital converter 32 contains an extremely high number of samples which are applied as an input to a Hilbert transform filter 34 of well-known construction. The Hilbert transform filter 34 performs two tasks which are to convert a real data input into a complex data output having real and imaginary components and to further greatly attenuate half of the wideband digital spectrum of the incoming digital signal. The output of the Hilbert transform filter 34 is applied to a frequency converter 36 which shifts the intermediate frequency input data after filtering by the Hilbert transform filter 34 to a lower carrier frequency. The inputting of digitally synthesized quadrature sinewaves from a digital frequency synthesizer 38 to the frequency converter 36 downwardly shifts the data in the same manner as described above in conjunction with FIG. 1. The particular specified frequency down to which the data outputted from the Hilbert transfer filter 34 is shifted is specified by the input FREQUENCY CONTROL WORD 40. As a consequence of the filtering function performed by the Hilbert transform filter 34 eliminating at least half the digital data bandwidth and by the frequency converter 36 downshifting the filtered data, the output from the frequency converter is applied to a down sampler 42 which eliminates the excess half of the data samples. The output of the down sampler is applied to a frequency shift 44 which shifts up the frequency of the output from the down sampler 42 by a frequency shift equal to one quarter of data sampling rate F.sub.s. A frequency shift controller 45 produces control signals SWITCH RAILS, NEGATE Q and NEGATE I which operate in accordance with the relationship set forth in the table below to produce a cyclical output of +1, +j, -1 and -j which is clocked at the data sampling rate F.sub.s. The circuitry for generating the control signals is discussed below in conjunction with FIG. 5.
Multiplier Control Signals +1 do not negate, nor swap rails +j negate Q-rail, swap rails (i.e. I and Q are switched) -1 negate both rails, do not swap rails -j negate I-rail, swap rails
The frequency shift 44 outputs I and Q data signals which are shifted to a correct frequency position to align the I and Q data in frequency with channels to be produced by a channelizer within digital demodulator processing 46 in a manner like FIG. 1.
For spectral efficiency and to prevent aliasing due to the down sampler 42, the digital spectrum produced by the frequency conversion 36 has channels positively and negatively spaced about baseband but does not have a channel at baseband. A channelizer requires alignment of the input data with a channel centered at baseband in order to function properly. The frequency shift 44 performs a frequency shift equal to one quarter of the data sampling rate F.sub.s where F.sub.s is the data sample rate of data outputted from the down sampler 42.
The demodulator containing the digital tuner 30 of FIG. 2, like the demodulator 10 of FIG. 1, suffers from not being readily testable for malfunctions of the demodulator signal processing downstream of the tuner from a remote location.